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A multi-band HF loop antenna

Amateur radio has 10 HF (shortwave) bands, from 1.8 MHz (160 meters) through 28 MHz (10 meters). The different bands are great for talking to amateur radio operators (hams) at different distances, different times of the day, different seasons and different times during the solar cycle. Only one problem: most hams do not have space in their yard for 10 different antennas. Fortunately it is possible to build one antenna that can be used effectively on most of the bands, for example this 80 meter loop antenna.

The first myth that we need to let go of is that we need a very low SWR on every band for an antenna to work well. On the HF bands, when using a decent feedline (eg. RG8 or LMR-400 coax, or ladder line), we can get away with a 4:1 SWR and still got low feedline losses. For example, a 3:1 VSWR at 18.1 MHz in 100 feet of LMR-400 coax leads to a loss of only 0.8 dB. You can plug in other frequencies, SWR values and feedline specifications in the coax calculator to find out what kind of SWR values will be acceptable to you. Antenna tuners can deal with these SWR values just fine, too.

Now that we have established that low single digit SWR values are no problem for losses, the goal for a multi-band antenna becomes clear. Instead of trying to get the SWR as low as possible on one band, the goal is to get the SWR "low enough" for as many bands as possible. Additionally, the higher bands have more feedline losses than the lower bands. A consequence of this is that if we have to accept a higher SWR on some bands to get the SWR lower on other bands, we should aim to have the higher SWR values on the lower frequencies and the lower SWR values on the higher frequencies.

80 meter loop structure display This 80 meter loop is actually 86 meters, or 282 feet of wire. This is a little long for 80, 17 and 12 meters, but a little short for 40 and 30 meters and just right for 20, 15 and 10 meters. If we make the loop much longer or shorter, we would end up with the antenna being unsuitable for some of the bands. At this length, it is a good compromise for all of the bands.

In practice, the shape of the loop will be determined by the location of good supports for the loop (usually trees). The shape of the loop is not critical at all, my loop is an irregular pentagon and it seems to get out fine. This model is square, however the feedpoint has been placed in an odd location. This avoids the typical patterns of a loop fed at a corner or in the middle and results in the kind of irregular looking patterns you would expect from a real life loop antenna.

Feedpoint impedances of the 80 meter loop on different bands

Band

Low edge

Center

High edge

160 meters

587 -j8605

198 -j4497

113 -j3025

80 meters

123 -j82

140 +j63

184 +j245

60 meters

3000 -j1018

2757 -j1371

2498 -j1566

40 meters

119 -j162

104 -j69

106 +j40

30 meters

200 -j440

203 -j428

200 -j415

20 meters

220 -j86

229 -j8

246 +j75

17 meters

280 +j109

290 +j130

300 +j150

15 meters

252 -j61

264 +j19

290 +j97

12 meters

283 +j94

290 +j112

298 +j130

10 meters

268 -j66

378 +j207

800 +j239

80 meter loop frequency plot The table above makes it fairly easy to choose the feedpoint impedance for the 80 meter loop antenna. The lower bands (80 and 40) want around 100 ohm, 30 and 20 want a 200 ohm feedpoint and 17 through 10 meters would be happiest with a 300 ohm feedpoint. Going with the middle value, 200 ohms, looks like it should provide a reasonable SWR for every band. The frequency chart confirms that using a 200 ohm feedpoint gives us a workable SWR on every band. The gain figures from the graph look reasonable, too.

Matching 160 and 60 meters is out of the question with this antenna, so lets forget about them. We can always build a compact 160 meter antenna for those bands.

Gain and radiation patterns

We know the antenna will load up on 80 and 40 through 10 meters and that the maximum gain is acceptable on all the bands. However, some questions remain: does it radiate in a useful direction and at a useful angle? To get the answer to these questions, we will examine the radiation patterns of the 80 meter loop on all the bands.

Gain and takeoff angle of an 80 meter loop at 15 meters high

Band

Maximum gain

Takeoff angle

80 meters

7.6dBi

90 degrees

40 meters

6.9dBi

40 degrees

30 meters

9.7dBi

30 degrees

20 meters

11.7dBi

20 degrees

17 meters

11.1dBi

16 degrees

15 meters

10.8dBi

10 degrees

12 meters

12.9dBi

10 degrees

10 meters

11.8dBi

10 degrees

The table suggests the antenna is good for regional contacts on 80, and good for DX contacts on the higher bands. If we look at the radiation patterns in some more detail we will see that the pattern on 80 and 40 is still acceptable for DX. In fact, from my location in the north-eastern USA, I have managed to make 80m voice contacts with most of Europe and some South American and African countries. At a DX angle of 30 degrees, this antenna has a gain of between -1.5dBi and 1.2dBi on 80 meters. Not great, but workable when conditions are nice.

On the higher bands the antenna is a very nice DX antenna when mounted at 15 meters high. Mounting the antenna higher will likely decrease the angle of incidence on 30 and 40 meters, making the antenna more of a DX antenna on those bands. On the other hand, lowering the antenna is likely to raise angles on 40, 30 and 20 meters, which makes the antenna a little less of a DX antenna. Without further ado, here are the radiation patterns of the antenna on the different bands:

The patterns look quite dramatic on the higher bands, but at low angles there is a decent amount of radiation in most directions. After 3 months of use and over 700 QSOs, I have not figured out yet where exactly the nulls are in my antenna.

Practical notes on how to build this antenna

The antenna can be hung from any handy support, like trees, antenna towers or your house. Always be sure the antenna wire does not cross electrical wiring, as that could pose a serious safety hazard! My antenna is hanging from 5 trees, with the antenna wire being attached to a dipole center insulator and balun at one tree.

Insulator for the loop antennaplenty of rope at the pine tree At the other 4 trees, the antenna wire can move freely through egg insulators, like the one shown. This free movement, as well as some slack, is important as the wind will sometimes move the treetops around by a few meters. During one storm, it looked like the top of one of the antenna support trees moved as much as 5 feet off center, or almost 3 meters total side to side!

By giving the antenna enough slack and freedom of movement at the insulators, it survived the storm with 50-60mph winds.

feed point with balun I feed my antenna with good 50 ohm coax (LMR-400) and a 4:1 balun. Remember that while the SWR on most bands is under 2:1, we have a few bands and band segments where we operate with an SWR up to 5:1. The higher SWR, especially when the load is reactive, can lead to higher voltages or currents in the balun. If you buy a 100W balun and run 100W into it with a 5:1 SWR, you could damage the balun, your radio, or both. To be on the safe side, I bought a 2kW balun for this loop antenna. This despite the fact that I only run 100W from my transceiver and do not have an amplifier. The oversized balun seems to hold out well, even on the frequencies where the SWR is higher. A 2kW 4:1 balun only costs $10 more than a lower power balun, so there really is no excuse to not play it safe.

With the amount of feedline here (about 130 feet), the maximum feedline loss on the frequencies I use is around 1.2dB, with the typical loss being lower than that. On all the frequencies I use, the coax calculator says that between about 80% and 95% of the power makes it to the antenna. If your loop antenna is further away from your house, you may want to feed it with ladder to keep feedline losses to a minimum.

The installation at my home

coax to the feedpointcoax entry into the house One criteria for the antenna at my house is that it is not too visible. From the living room or from the street, you can barely see the coax running up the side of the tree, because I have clamped it to the back of the tree with coax staples. To get the coax outside, I added an electrical box identical to the one that was already there, and buried the coax from the entry point to the tree.

To protect the now easily forgotten coax from gardening tools and rodents, it is buried in 1" PVC pipe.

support rope in flagpole stylenear the top of the tree By running the support rope around the tree in a loop, like is done with the rope on a flagpole, I can easily raise and lower the antenna. Having this ability is important, since the antenna is so close to the top of the tree that it would be impossible to climb up there to unsnag something. Depending on the height of the antenna and the distance between the supports, it may be necessary to lower the antenna at two or more points, just to get to one support rope.

Conclusions

While this loop antenna will not outperform a yagi or a 4 square, it is a decent all-around antenna. The main advantages of this 80 meter loop are the low takeoff angle on higher bands (good DX antenna) and the fact that it can be easily matched on 8 HF bands. In fact, my 80 meter loop antenna could be tuned on 6 of the 8 bands with just the built-in ATU in my Kenwood TS-930SAT. Adding an LDG AT-200 Pro autotuner allowed it to be tuned up on all 8 bands.

I use xnec2c for my antenna simulations, but the NEC file below should also work with other NEC2 derivatives, eg. cocoaNEC.